1SSL_CTX_set_tlsext_ticket_key_cb(3) OpenSSLSSL_CTX_set_tlsext_ticket_key_cb(3)
2
6 SSL_CTX_set_tlsext_ticket_key_cb - set a callback for session ticket
7 processing
8
10 #include <openssl/tls1.h>
11 long SSL_CTX_set_tlsext_ticket_key_cb(SSL_CTX sslctx,
13 int (*cb)(SSL *s, unsigned char key_name[16],
14 unsigned char iv[EVP_MAX_IV_LENGTH],
15 EVP_CIPHER_CTX *ctx, HMAC_CTX *hctx, int enc));
16
18 SSL_CTX_set_tlsext_ticket_key_cb() sets a callback fuction cb for
19 handling session tickets for the ssl context sslctx. Session tickets,
20 defined in RFC5077 provide an enhanced session resumption capability
21 where the server implementation is not required to maintain per session
22 state. It only applies to TLS and there is no SSLv3 implementation.
23 The callback is available when the OpenSSL library was built without
25 OPENSSL_NO_TLSEXT being defined.
26 The callback function cb will be called for every client instigated TLS
28 session when session ticket extension is presented in the TLS hello
29 message. It is the responsibility of this function to create or
30 retrieve the cryptographic parameters and to maintain their state.
31 The OpenSSL library uses your callback function to help implement a
33 common TLS ticket construction state according to RFC5077 Section 4
34 such that per session state is unnecessary and a small set of
35 cryptographic variables needs to be maintained by the callback function
36 implementation.
37 In order to reuse a session, a TLS client must send the a session
39 ticket extension to the server. The client can only send exactly one
40 session ticket. The server, through the callback function, either
41 agrees to reuse the session ticket information or it starts a full TLS
42 handshake to create a new session ticket.
43 Before the callback function is started ctx and hctx have been
45 initialised with EVP_CIPHER_CTX_init and HMAC_CTX_init respectively.
46 For new sessions tickets, when the client doesn't present a session
48 ticket, or an attempted retreival of the ticket failed, or a renew
49 option was indicated, the callback function will be called with enc
50 equal to 1. The OpenSSL library expects that the function will set an
51 arbitary name, initialize iv, and set the cipher context ctx and the
52 hash context hctx.
53 The name is 16 characters long and is used as a key identifier.
55 The iv length is the length of the IV of the corresponding cipher. The
57 maximum IV length is EVP_MAX_IV_LENGTH bytes defined in evp.h.
58 The initialization vector iv should be a random value. The cipher
60 context ctx should use the initialisation vector iv. The cipher context
61 can be set using EVP_EncryptInit_ex. The hmac context can be set using
62 HMAC_Init_ex.
63 When the client presents a session ticket, the callback function with
65 be called with enc set to 0 indicating that the cb function should
66 retreive a set of parameters. In this case name and iv have already
67 been parsed out of the session ticket. The OpenSSL library expects that
68 the name will be used to retrieve a cryptographic parameters and that
69 the cryptographic context ctx will be set with the retreived parameters
70 and the initialization vector iv. using a function like
71 EVP_DecryptInit_ex. The hctx needs to be set using HMAC_Init_ex.
72 If the name is still valid but a renewal of the ticket is required the
74 callback function should return 2. The library will call the callback
75 again with an arguement of enc equal to 1 to set the new ticket.
76 The return value of the cb function is used by OpenSSL to determine
78 what further processing will occur. The following return values have
79 meaning:
80 2 This indicates that the ctx and hctx have been set and the session
82 can continue on those parameters. Additionally it indicates that
83 the session ticket is in a renewal period and should be replaced.
84 The OpenSSL library will call cb again with an enc argument of 1 to
85 set the new ticket (see RFC5077 3.3 paragraph 2).
86 1 This indicates that the ctx and hctx have been set and the session
88 can continue on those parameters.
89 0 This indicates that it was not possible to set/retrieve a session
91 ticket and the SSL/TLS session will continue by by negiotationing a
92 set of cryptographic parameters or using the alternate SSL/TLS
93 resumption mechanism, session ids.
94 If called with enc equal to 0 the library will call the cb again to
96 get a new set of parameters.
97 less than 0
99 This indicates an error.
100
102 Session resumption shortcuts the TLS so that the client certificate
103 negiotation don't occur. It makes up for this by storing client
104 certificate an all other negotiated state information encrypted within
105 the ticket. In a resumed session the applications will have all this
106 state information available exactly as if a full negiotation had
107 occured.
108 If an attacker can obtain the key used to encrypt a session ticket,
110 they can obtain the master secret for any ticket using that key and
111 decrypt any traffic using that session: even if the ciphersuite
112 supports forward secrecy. As a result applications may wish to use
113 multiple keys and avoid using long term keys stored in files.
114 Applications can use longer keys to maintain a consistent level of
116 security. For example if a ciphersuite uses 256 bit ciphers but only a
117 128 bit ticket key the overall security is only 128 bits because
118 breaking the ticket key will enable an attacker to obtain the session
119 keys.
120
122 Reference Implemention:
123 SSL_CTX_set_tlsext_ticket_key_cb(SSL,ssl_tlsext_ticket_key_cb);
124 ....
125 static int ssl_tlsext_ticket_key_cb(SSL *s, unsigned char key_name[16], unsigned char *iv, EVP_CIPHER_CTX *ctx, HMAC_CTX *hctx, int enc)
127 {
128 if (enc) { /* create new session */
129 if (RAND_bytes(iv, EVP_MAX_IV_LENGTH) ) {
130 return -1; /* insufficient random */
131 }
132 key = currentkey(); /* something that you need to implement */
134 if ( !key ) {
135 /* current key doesn't exist or isn't valid */
136 key = createkey(); /* something that you need to implement.
137 * createkey needs to initialise, a name,
138 * an aes_key, a hmac_key and optionally
139 * an expire time. */
140 if ( !key ) { /* key couldn't be created */
141 return 0;
142 }
143 }
144 memcpy(key_name, key->name, 16);
145 EVP_EncryptInit_ex(&ctx, EVP_aes_128_cbc(), NULL, key->aes_key, iv);
147 HMAC_Init_ex(&hctx, key->hmac_key, 16, EVP_sha256(), NULL);
148 return 1;
150 } else { /* retrieve session */
152 key = findkey(name);
153 if (!key || key->expire < now() ) {
155 return 0;
156 }
157 HMAC_Init_ex(&hctx, key->hmac_key, 16, EVP_sha256(), NULL);
159 EVP_DecryptInit_ex(&ctx, EVP_aes_128_cbc(), NULL, key->aes_key, iv );
160 if (key->expire < ( now() - RENEW_TIME ) ) {
162 /* return 2 - this session will get a new ticket even though the current is still valid */
163 return 2;
164 }
165 return 1;
166 }
168 }
169
171 returns 0 to indicate the callback function was set.
172
174 ssl(3), SSL_set_session(3), SSL_session_reused(3),
175 SSL_CTX_add_session(3), SSL_CTX_sess_number(3),
176 SSL_CTX_sess_set_get_cb(3), SSL_CTX_set_session_id_context(3),
177
179 This function was introduced in OpenSSL 0.9.8h
1801.0.2o 2020-08-01SSL_CTX_set_tlsext_ticket_key_cb(3)